Club 80-90 Forums

A while ago I noticed that the top-up tank was empty, so filled it up. As soon as I went for a drive it sucked all the coolant out of there and overflowed the main expansion tank (not sure where it was coming out of, but forcing its way out somewhere).

I replaced the expansion tank cap, thinking that the valves in that must be faulty, but now it is doing it again. Bled the radiator, engine as per Haynes manual, but that hasn't made a difference.

Am I right in thinking that there are two valves in the cap - for taking coolant in when pressure is too low and allowing it out when it is too high? Are my new cap's valves just getting gunked up with dirt in the coolant system?

Have a look at the tank in the engine bay it has a sensor on the top of it.
it could be leaking from there

is it a genuine VW cap?

no - it is a Just Kampers one, does look slightly different underneath, but fits nicely.

Could be a faulty cap, but could be more sinister... check to see if there are any leaks around the system... if not and its running normal temps etc but its blowing the coolant out of the tank, then it could be a head stud/head problem.. What engine do you have..

OK, assuming it's a DG/DJ engine .... Our DG header tank (from JK) had a split which was only visible when the engine was warm. I sent it back and received an F.O.C. replacement.

The screw-in level sensor can crack in the tank allowing coolant out above the o-ring seal, possibly due to over-tightening or a faulty batch of sensors.

Look for cracks around the neck of the tank near the cap and deposits around the sensor.

Cheers,Ian.

The great thing about this forum is that there is little that hasn't been seen before as I find out.
Arrived back from a week in Northern Ireland (a great place) to find steam coming out the old air intake on the side.
Turns out to be a smal 1/2" crack in the expansion bottle part way between the cap and the sensor. Only leaks under pressure and dumps onto the exhaust.

Proud owner of new expansion bottle , and then a leak around the sensor .. but a new O ring sorted that one.

The wiki on venting was useful, especially the go for coffee and do it all again .. took 3 goes for me.

It is a DG engine, I've opened up the engine compartment when it has been leaking and it isn't coming out of the sensor at the top, or from around the cap. Must be forcing its way out of one of the seams at high pressure.

But the expansion tank is full to the brim - shouldn't the extra coolant end up in the top-up tank?

Not if the cap, sensor or tank are faulty...

It definately sounds like a cracked coolant level sensor. on a dg you might have to run the engine and throttle up on the cable cam on the carb to get sufficient pressure to find the coolant pissing out of the cracked sensor. Don't be shy. That's what happened to me anyway.

Definitely not coming out of sensor - and the sensor doesn't control the pressure cap so I can't see how the sensor can be at fault.

According to a workshop manual I have the pressure cap should go both ways (open at high pressure to let excess coolant out, and open at low pressure to let more coolant in) I am beginning to think that the pressure cap JK sent me is one way, which means all the coolant ends up in the system and forces its way out of the seam on the expansion tank when pressure rises.

Anyone else had this problem?

Have you eliminated leaks from the expansion tank being the prob?, and you are right, the cap is pressure and vaccuum sensitive. The idea is that if the pressure gets too high in the system then to save popping a pipe off it overflows into the top up tank behind the numberplate. This relieves the system of exccess pressure. Any air pockets in the system should end up in the expansion tank. As this hot air ( and the coolant generally) cools, it forms a vacuum which then sucks coolant back up from the top up tank, keeping the expansion tank full.

If the top up tank is filling up then it is due to two reasons.
1. The cap is faulty.
2. There is excess pressure in the system.

If the expansion vessel is not filling up when the system cools it is also due to two possible reasons.
1. The cap is faulty.
2. There is insuffiecent vacuum.

Number one is easy to fix, new cap.

Number two's are always harder and more time consuming. Could be more sinister as toomanytoys says and mean it's head gasket time.
You can have a garage pressure test the sysytem and see what is going on. You can also test the coolant for combustion gasses and see if the heads are foobah. The cap is a common problem and is worth checking.
The top up tank must have a vent on it. this is on the cap, dont over tighten it. If the ccoolant was nice and clean and is now a bit mucky then poss head gasket.

Simon.

Just been through all this. I removed the two screws , pushed exp tank forward, and it was splitt allong the seam at back. Only leaked under pressure, In Traffic n stuff. New 1 from VW £23.

funbus 1 wrote:Just been through all this. I removed the two screws , pushed exp tank forward, and it was splitt allong the seam at back. Only leaked under pressure, In Traffic n stuff. New 1 from VW £23.

As above.

Just Kampers one lasted 9 months.
Not had a chance to send it back yet. VW one much better quality.
T

Thanks for the advice, I've ordered a new VW expansion tank for £27 which includes a new cap, instead of a JK one for £25 which doesn't.